CHAPTER X 

 DIGESTION 



THE development of energy by animals is based, as 

 we have seen, upon the combustion of food materials. 

 In order to bring food to the state in which it can be 

 utilized by their tissues for the production of energy, it 

 must be made soluble by means of digestion. 



Simplest form of digestion. The process of digestion 

 may be very simple or extremely complex. In the ameba, 

 a particle of food is taken into its body apparently at 

 any point. The nutritious part is then dissolved through 

 the action of certain peculiar substances called enzymes, 

 which are found in the protoplasm. Thus dissolved, it 

 is distributed throughout the body by the flowing and 

 mixing of the protoplasm itself. The part of the food 

 which cannot be dissolved is finally pushed out through 

 the side of the body. 



More complex forms. In animals which consist of 

 many cells, a special opening, or mouth, is found through 

 which alone food can enter. Connected with this is a 

 special canal running through the body, within which 

 all undissolved food material is kept. This canal has a 

 definite wall lined with cells, which secrete substances 

 capable of dissolving the food materials. It ends in an 

 opening through which all the undigested parts of the 

 food are expelled. This canal for food, the alimentary 

 canal, is really an infolding of the outside surface of the 

 body, for the double purpose of furnishing a reservoir 



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